Challenging week! A customer came to me with a request to design an Underfloor Air Distribution System. I had never designed this type of system before except while working as a design build engineer where I "red lined" several to the schematic design level. Good thing those systems were never built because the design procedures have changed since the scheme was introduced to engineers in America. Not sure why the same problems never occurred in systems in Europe where the scheme was developed but the Center for the Built Environment has introduced a rather simple design procedure based on experimental data.
Underfloor air distribution pressurizes the area under a raised floor causing air delivery to occur using high induction diffusers in the floor to condition air in the occupied zone, usually 67" AFF. The people and the equipment are cooled by convection of air as it moves up the occupants or surface by thermal buoyancy. The heated air continues to rise gaining additional heat as it rises past the lights and other loads above the 67" height. The benefit of the system is instead of contaminates (viruses, bacteria, pollutants, gases) mixing throughout the space they rise out of the occupied zone causing fewer sick days and higher productivity of the building occupants. Other benefits include better system efficiency because the fans require less static pressure and the less energy usage because the air is supplied at 65F instead of 55F.
In designing the system I read the design guides provided by the Center for the Built Environment and Price. I also had some help from my local Price sales representative. Based on the reading and recommendations I broke out the perimeter loads from the space and calculated the air required at the perimeter using a supply air temp of 65F and a space set point of 79F. I then selected higher velocity diffusers to wash the walls with cool air that will (hopefully) travel up the wall and out of the space taking the heat with it. The interior loads I calculated the total load including lights and equipment and multiplied it by a factor of 0.65 and calculated the air flow using the same 14 degree delta T I used to calculate the perimeter air flows. I then selected a round swirl air diffuser to deliver air to the space.
The best part about the project is I learned some new information and increased my knowledge base. Another consultant and I also worked on selecting water cooled air handling units that will utilize a ground source well field to reject and absorb heat. More on that latter.....
Following is a Youtube video I found discussing underfloor air distribution and some of the other advantages. Skip to 2 minutes 43 seconds to skip to the underfloor discussion.
